A method of visualizing image information, via an electrostatic latent image, such as an electrophotographic type is currently used in various fields, such as the copying machine or the printer, with development of its technology and enlargement of market demand. Particularly, in recent years, demands for environmental response and cost reduction are increased, so that a toner consumption reducing technique has become very important. This technique is important also from the viewpoint of reduction in energy generated in a process in which the toner is fixed on the recording material. Particularly, in the image forming apparatus using the electrophotographic type for office use, the technique plays an important part also from a demand for energy saving.
On the other hand, with development of digitalization and colorization, the image forming apparatus of the electrophotographic type has started to be applied to some print fields; for example, commercialization of the image forming apparatus has started to become conspicuous in the fields of graphic arts and short-run printing such as print on-demand (POD). In the case where the entry of the image forming apparatus into the POD market is taken into consideration, the electrophotographic type has a feature of an on-demand property as digital to press printing, but involves many problems related to a color reproduction region, texture, image stability and media compatibility, in order to have market value as an output product. While addressing these problems, also from the viewpoint that awareness of the above-described cost reduction is raised and thus a cost per sheet of the output product is suppressed at a low level, the toner consumption reducing technique becomes important.
Here, with respect to the toner consumption reducing technique, the following proposals have been made.
In Japanese Laid-Open Patent Application (JP-A) 2004-295144, an absolute value of a charge potential of a photosensitive member is set at a low condition such as 350-550 V, and a toner having a high coloring power of 0.3-0.7 mg/cm2 is used so that a toner amount on a transfer material after transfer can ensure a necessary image density after fixing. By this, downsizing, low cost and high-speed printing can be effected, so that a full-color image is stably formed over a long term.
In JP-A 2005-195670, as the toner, the following crystalline toner is used. That is, the crystalline toner of 0.02 log(Pa)/° C. or less in gradient of storage elastic modulus to temperature at Tm+20° C. to Tm+50° C. and 5 μm or less in weight-average particle size is used. Further, the crystalline toner of 0.35 mg/cm2 or less in monochromatic toner maximum amount per unit area is transferred as the toner image onto a transfer sheet of 20 or more in texture index and then the toner image is fixed in a fixing time from 50 ms to 500 ms. By this, an image reduced in image non-uniformity such as non-uniformity of toner amount per unit area, uneven glossiness and color difference non-uniformity is formed.
In JP-A 2005-195674, as the toner, the following crystalline toner is used. That is, the crystalline toner of 5μ or less in weight-average particle size and 0.02 log(Pa)/° C. or less in gradient of storage elastic modulus to temperature at Tm+20° C. to Tm+50° C. is used. Further, the crystalline toner of 0.35 mg/cm2 or less in monochromatic toner maximum amount per unit area is transferred as the toner image onto a transfer sheet and then the toner image is fixed at a fixing roller surface temperature of 130° C. or less. By this, a color image with less image roughening and excellent in fixing property can be formed.
The conventional toner consumption reducing technique, i.e., a method of enhancing the coloring power of the toner itself in which the toner consumption (amount) is reduced and at the same time, the toner is reduced in particle size can obtain a necessary image density while reducing the toner consumption. However, a new problem resulting from the reduction itself of the toner consumption has arisen.